Figure 5.

Models for the complex of the large β ring and ssDNA (a) and the β helix formed on dsDNA (b). The ssDNA is believed to wrap around the outside of the 18-subunit rings, as proposed for phage P22 erf protein rings (16). The exposure of the bases would provide the catalytic role played by these rings in DNA annealing. The helical filaments are formed by β protein on preformed dsDNA, or on dsDNA products of the β-catalyzed annealing reaction. The β filament induces a left-handed supercoil in the DNA, with about one supercoil turn for every 100 bp.